Conical spring washers (Belleville washers) are used in fastening systems to assure that a compressive force is maintained on the clamped members throughout the intended service cycle. Once compressed the conical spring washer continuously produces a force exerted on its immediately adjoining members (a separating force). When a conical spring washer is compressed between a bolt head or nut securing a clamped member and the clamped member, the separating force of the conical spring washer assists in clamping the clamped member. The spring washer's effective operating travel produces and maintains a force on clamped members when there is differential movement (strain) between and among the clamped members and the clamping member, e.g., a bolt.
There is an expectation that the effective operating travel (available deflection) and corresponding force needed to fully compress the conical spring can always be completely recovered when the washer is not compressed. In practice, the effective operating travel (or deflection) is limited because the compressive hoop stress (the bending stress at the convex inner edge) of the washer ("C" - FIG. 4) typically exceeds the compressive yield strength of the washer material at very small deflection values. Likewise, the tensile hoop stress (the bending stress at the outer edge) ("T") typically exceeds the tensile strength of the washer material at these small deflections. If either or both of these situations occur, the washer material reaches the plastic state; the washer does not return to its original form and loses some or all of its spring properties.
Ideally Belleville spring washers are used to maintain a predefined clamping force on the clamped members within a predetermined range of differential motion of the member on each side of the washer. Alternatively, Belleville washers are also used in applications where they are initially flattened but become the predominant clamping force when the clamped and clamping members experience conditions that cause them to lose their initial stress (for example when the members are subjected to vibration and/or to temperature changes and thermal gradients causing differential expansion and/or contraction of members).
The large thermal cycles and gradients (approx. 300.degree. C.) between components experienced in processing semiconductor wafers (at normal temperatures of 400.degree.-500.degree. C.) can and do cause fastening systems unable to accommodate the differential movement to fail (e.g., causing loss of electrical conductance and/or loss of vessel pressure integrity).
An increase in the range of the fastening system is needed to avoid these failures, and to support processes that operate faster and therefore generate potentially larger thermal cycles and gradients between clamped and clamping members than the existing processes. The spring (elastic) properties of the washer must be present throughout its available deflection range and stresses causing the washer to plastically deform must be avoided.